In recent years there have been suggested improved methods of building double walled storage tanks used primarily for storing liquids. The double walled tanks are desired, and recently mandated, to ensure adequate containment of stored liquid. Leakage through one wall will not immediately cause an environmental problem since the second wall should then serve to contain it. However, to be effective, any wall damage sufficient enough to allow leakage must be detectable so that it can be remedied. It is important that means be provided between the double walls that will alert the owner/operator of the tank of any wall failure.
U.S. Pat. Nos. 4,561,292, 4,640,439, 4,644,627, 4,744,137, 4,780,946, 4,780,947 and 4,844,287 disclose double walled storage tank systems wherein means are used to maintain a spaced relationship between the walls of the tanks. The spaced relationship is needed for effective leak detection purposes. However, the spacing means suggested can require a separate time consuming step in properly positioning some specially constructed material on a tank wall prior to forming an outer tank wall. U.S. Pat. Nos. 4,859,262 and 4,871,078 disclose double walled storage tank systems which also use special materials, though the systems are more efficiently built.
U.S. Pat. Nos. 4,989,447 and 5,000,342 disclose still other double walled storage tank systems. A separating material is used between the walls of the disclosed systems to ensure leakage detection capability. The disclosed systems also recognize the advantage of structurally bonding or tieing the walls together for strengthening purposes. For example, U.S. Pat. No. 4,989,447 teaches the use of a porous material such as a needled non-woven fibrous material between the walls. A binder saturates the porous material so as to bond the fibers together and to the inner and outer walls of the tank system. The binder present in the porous material bonds the fibers together in a manner which permits wicking of any leakage. U.S. Pat. No. 5,000,342 uses a special separating material having spaced holes extending through it. Formation of an outer wall of the tank with a liquid uncured resinous material also fills the holes which upon cure forms resin attachment columns. The columns are attached at one end to the inner tank and at the other 20 end to the outer wall. The latter described tank system in particular is economically feasible and provides an integrally strengthened tank system.
There has now been developed methods whereby new and used storage tanks can be provided with a secondary containment wall having a minimal closed communication space therebetween. The resultant double walled tank systems are capable of withstanding elevated pressure forces between the walls without suffering damage.